The present invention relates to an improved process for the recovery of uranium as a by-product of phosphate fertilizer production. More particularly, it relates to a process for the recovery of uranium from the wet-process phosphoric acid that is produced in the treatment of phosphate rock found in Florida and other states of the United States and in several foreign countries. The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.
The main purpose in mining phosphate rock is to recover a high-phosphate-containing fertilizer. The first step in winning the phosphate from the rock consists of digesting the rock with sulfuric acid under controlled temperature and concentration conditions to produce a phosphoric acid solution and an insoluble calcium sulfate (gypsum). Prior to further treatment to produce a commercially acceptable fertilizer product, this phosphate solution can be processed to remove valuable uranium values provided such processing is economically justified.
It is accordingly an object of this invention to provide a process for the recovery of uranium values from uraniferous phosphate mined deposits that satisfies this criterion.
The phosphate rock mined in Florida contains 0.01 to 0.02 weight percent uranium or from 0.2 to 0.4 pound of uranium per mined ton. While this appears to be a relatively small concentration, it represents a large supply of uranium because of the large amount of phosphate rock processed. For example, it has been estimated that in 1970 the wet-process phosphoric acid produced from Florida phosphate rock .[.will have.]. .Iadd.contained .Iaddend.about 2,000 tons of U.sub.3 O.sub.8 dissolved therein, representing a significant potential source of uranium.
The so-called "wet-process" phosphoric acid solution resulting from the acidulation of phosphate rock has the following approximate analysis (concentration in grams per liter): 600 H.sub.3 PO.sub.4, 0.17 uranium, 9 iron, 1 calcium, 30 sulfate, and 27 fluorine. This solution can serve as the aqueous feed for a liquid-liquid solvent extraction process for uranium recovery. In recent years, workers have demonstrated the technical feasibility of solvent extraction as a way to recover uranium from wet-process phosphoric acid. .Iadd.Some of the earlier teaching concerned with uranium extraction are found in ORNL-2259, "Synergistic Uranium Extractants: Combination of Neutral Organophosphorous Compounds with Dialkylphosphoric Acids" by Blake et al; in ORNL-1903, "The Extraction and Recovery of Uranium (and Vanadium) from Acid Liquors with Di(2-ethyl-hexyl) phosphoric Acid and some Other Organophosphorus Acids" by Blake et al; and in ORNL-2952, "Recovery of Uranium from Di(2-ethylhexyl) phosphoric Acid (Dapex) Extraction with Ammonium Carbonate" by the applicants. .Iaddend.One of the latest efforts is described in ORNL-TM-2522, "Solvent Extraction of Uranium from Wet-Process Phosphoric Acid," by F. J. Hurst et al., .[.a publication.]. .Iadd.(publications .Iaddend.of the U.S. Atomic Energy Commission.Iadd.).Iaddend..[.. In that document, a flow sheet is described in which.]. .Iadd.wherein .Iaddend.an aqueous uranium-containing phosphate feed solution is contacted with a synergistic extractant combination .Iadd.of dialkylphosphoric acid and trialkylphosphine oxide, and more specifically an extraction combination .Iaddend.consisting of di(2-ethylhexyl)phosphoric acid (D2EHPA) and trioctylphosphine oxide (TOPO) dissolved in an organic diluent to effect extraction of uranium in the +6 oxidation state. .Iadd.The synergistic enhancement of the extraction coefficient due to the addition of TOPO was greatest for D2EHPA and other dialkyl phosphoric acids. .Iaddend.While this process represented a significant advance in this technology, it nevertheless suffers from several disadvantages: (1) Appreciable amounts of phosphate and iron are co-extracted into the organic phase and follow the uranium into the stripping solution where they complicate recovery of the uranium product. (2) The use of ammonium carbonate as the stripping reagent results in the conversion of the D2EHPA to a highly hydrated ammonium salt. When the stripped solvent is recycled to the extraction circuit, the extracted water transfers to the aqueous phase and dilutes the wet-process phosphoric acid. This in turn requires additional cost to effect evaporation and reconcentration of the wet-process phosphoric acid. (3) Humic acids are co-extracted from the acid phosphate feed by the organic extractant. These substances form stable complexes with metal ions, both with uranium and other metallic contaminants, to produce problems of phase separation and hindering precipitation of uranium. It is therefore an additional object of the present invention to reduce or eliminate the disadvantages of the process described in ORNL-TM-2522, the disclosure of which is hereby incorporated by reference.